ringhash: normalize uppercase in requestHashHeader from service config

With gRFC A76 (grpc#8159), when requestHashHeader is specified from the
service config it fails the validation since MD keys with uppercase
letters are normalized to lowercase. We should normalize the parsed
value before validation.

Author: yousukseung

xds/internal/balancer/ringhash/config.go

@@ -73,6 +73,7 @@ func parseConfig(c json.RawMessage) (*LBConfig, error) {
 		cfg.RequestHashHeader = ""
 	}
 	if cfg.RequestHashHeader != "" {
+		cfg.RequestHashHeader = strings.ToLower(cfg.RequestHashHeader)
 		// See rules in https://github.com/grpc/proposal/blob/master/A76-ring-hash-improvements.md#explicitly-setting-the-request-hash-key
 		if err := metadata.ValidateKey(cfg.RequestHashHeader); err != nil {
 			return nil, fmt.Errorf("invalid requestHashHeader %q: %v", cfg.RequestHashHeader, err)

xds/internal/balancer/ringhash/config_test.go

@@ -118,6 +118,16 @@ func (s) TestParseConfig(t *testing.T) {
 				RequestHashHeader: "x-foo",
 			},
 		},
+		{
+			name:                "request metadata key set with uppercase letters",
+			js:                  `{"requestHashHeader": "x-FOO"}`,
+			requestHeaderEnvVar: true,
+			want: &LBConfig{
+				MinRingSize:       defaultMinSize,
+				MaxRingSize:       defaultMaxSize,
+				RequestHashHeader: "x-foo",
+			},
+		},
 		{
 			name:                "invalid request hash header",
 			js:                  `{"requestHashHeader": "!invalid"}`,